Metal borohydrides containing BH4− complexes are attractive candidates for compact hydrogen storage because of their large hydrogen content relative to their weight and volume. Simple alkali metal or alkaline earth metal borohydrides such as LiBH4 (18.5 wt % hydrogen), NaBH4 (10.7 wt % hydrogen), Mg(BH4)2 (14.9 wt % hydrogen), and Ca(BH4)2 (11.6 wt % hydrogen) are too stable, requiring high temperatures (300-500° C.) to release hydrogen, and even partial re-absorption of hydrogen requires high H2 pressures. Moreover, intermediate phases such as Li2B12H12 can form during decomposition at high temperature, and once formed these phases strongly inhibit reversibility.
Transition metal borohydrides and mixed alkali metal-transition metal borohydrides are less stable, and therefore could be suitable candidates for hydrogen storage. Transition metal borohydrides include the borohydrides of chromium, cobalt, copper, iron, manganese, molybdenum, nickel, niobium, scandium, titanium, vanadium, yttrium, zinc, and zirconium. Mixed borohydride compounds of one of such transition metals and an alkali metal such as lithium, sodium, or potassium may also be formed. Unfortunately, in many cases the borohydride decomposes through the emission of diborane gas (B2H6) in addition to yielding hydrogen gas. Such emission is clearly deleterious; first, most of the available hydrogen is tied up as diborane rather than being released as H2, second, diborane evolution permanently removes boron from the material, and third, diborane may present environmental issues.
There is a need for a method of recovering hydrogen from transition metal borohydrides and mixed alkali metal-containing and transition metal-containing borohydrides while reducing or eliminating the evolution of diborane gas.